Single Sideband Signals for Phase Noise Mitigation in Wireless THz-Over-Fibre Systems

In future photonic-based THz backhaul links, integrating the optical local oscillator (LO) in a remote antenna unit can be beneficial in terms of optical bandwidth efficiency and higher compatibility with passive optical networks. In such a scenario, two approaches can be used to reduce the high phase noise associated with free-running lasers: 1) baseband (BB) signals & carrier recovery, and 2) single sideband (SSB) signals & envelope detection. In this paper, we compare the performance of the two approaches for various optical LO linewidths using 5 GBd 16-QAM signals. We find that, for a total linewidth wider than 0.55 MHz, the SSB approach yields better results. The superior performance, however, comes at the expense of reducing the net information spectral density (ISD) of the SSB signal by 39% compared to that of the BB signal. However, using signal-signal beat interference-mitigation algorithms, an ISD only 15% lower than the BB signal ISD was sufficient to meet the FEC requirement. Given these results, we believe that the envelope detection of SSB signals is a promising solution to mitigate the phase noise problem of THz links based on free-running lasers, without excessively compromising the spectral efficiency of the system.